Arrest That Chimp!

9 March 2009 (All day)

Mathias Osvath, Current Biology

Thinking ahead. Santino collected rocks and pieces of concrete (inset) long before he planned to hurl them at zoo visitors.

In 1997, at the Furuvik Zoo in Gävle, Sweden, a male chimpanzee named Santino began throwing stones at zoo visitors. Although Santino was clearly agitated each time, as evidenced by his forceful stomping around the compound and hair standing on end, the chimp didn't just grab the first thing he saw and launch it. Rather, observations over the past decade have shown that Santino spends the mornings before the zoo opens gathering the stones and organizing them into neat piles as a sort of ammunition store. The chimp's preparation suggests that apes can plan for future mental states--in this case anger--a cognitive talent once thought to be unique to humans.

Humans take planning for the future for granted. We shop for food even when we're not hungry, for example, because we know we will be hungry later. But until recently, scientists didn't know whether other animals thought ahead the same way. In the wild, many chimps use tools for procuring food, such as employing sticks to fish termites out of trees. But the apes tend to fashion the tools shortly before they are needed rather than far in advance. In the laboratory, on the other hand, carefully contrived experiments have shown that bonobos and orangutans can learn to choose and save the right tool that will later allow them to retrieve food.

Yet these lab experiments do not prove that animals can anticipate future emotional states. That's where Santino comes in. Soon after the only other male chimp in his group died and he was left alone with four females, Santino began hurling stones at zoo visitors. Sometimes he'd let fly with a "hail storm" of 10 or more projectiles per attack, according to one zookeeper. Although no one was hurt, zoo workers had to warn the crowd to keep back and usher Santino into his chimp house.

Primatologist Mathias Osvath of Lund University in Sweden has now collated a decades' worth of observations recorded by zookeepers that show Santino apparently knew he was going to be throwing mad and got ready for it in advance. While the zoo was closed and he was perfectly calm, the chimp would gather stones from an island within his enclosure. Later, he began adding pieces of concrete to his ammunition stores, which he fashioned into disk-shaped projectiles. Over the past decade, Santino has created hundreds of such caches, most of which were removed before the chimp had a chance to use them. In a report on the chimp's behavior published online today in Current Biology, Osvath concludes that because Santino was calm when collecting the ammunition but used it only when agitated, his behavior "is clearly identifiable as planning for a future [mental] state."

Although the study is limited to an individual and thus can't be generalized to a whole species, it is "extremely convincing," says Christophe Boesch, a primatologist at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany. Andrew Whiten, a psychologist at the University of St. Andrews in the United Kingdom, adds that the observations analyzed by Osvath are "compelling" and "quite special" because they bridge the gap between the contrived experiments in the laboratory and the inconclusive observations in the wild.